The invention relates to all possible friction clutch structures that are engaged in the rest position. A generally known structural form with which the method, according to the invention, can preferably be applied, is a dry clutch that can be engaged passively by the action of a contact pressure spring and disengaged and engaged by way of a pressure-medium-actuated or electric-motor- or electromagnetically operated clutch control element. Another possible structural form is a dry clutch that can be actively engaged by the contact pressure of a controllable contact pressure element which, in the rest condition, i.e., when the motor vehicle is parked with its drive engine switched off, is held engaged by a contact pressure element acted upon by energy, for example by compressed air from a pneumatic pressure reservoir. Likewise, the method can be used with a wet clutch that can be actively or passively engaged, in particular a disk clutch.
With such friction clutches, the fundamental problem arises of determining a torque characteristic M_K=f(x_K) which represents the transferable torque M_K of the friction clutch as a function of the regulating path x_K of an associated clutch control element, such as a hydraulic or pneumatic control cylinder or a transfer element connected thereto, such as a disengagement lever or release bearing, by way of which the desired transferable torque of the friction clutch is controlled.
Without any limitation of the invention to this type of structure, an example of a path-controlled dry clutch with spring contact pressure will be considered below, which can be disengaged and engaged by way of an associated clutch control element. In this case, the torque characteristic M_K=f(x_K) concerned describes the clutch torque M_K as a function of a regulating path x_K of the clutch control element that corresponds to the disengagement path. Since, as is known, the respective regulating positions x_K, associated with particular values of the clutch torque M_K, can vary depending on the operating temperature and the wear condition, in particular of the friction linings of the friction clutch. For exact actuation control of the friction clutch it is necessary to precisely determine a torque characteristic M_K=f(x_K) that corresponds to the situation at the time, i.e., to the momentary operating conditions.
For this purpose, it is known to determine at least two marker points M_K1(x_K1) and M_K2(x_K2) of the torque characteristic M_K=f(x_K), of which at least one marker point is determined at a slippage limit of the friction clutch. The entire torque characteristic M_K=f(x_K) can then be obtained by adapting an existing torque characteristic M_K′=f(x_K) to the currently determined marker points M_K1(x_K1) and M_K2(x_K2) by projection of the torque values M_K1(x_K1′) and M_K2(x_K2′) concerned.
In this case, until now at least one of the marker points has been determined at a slippage limit in that with the friction clutch engaged, the transmission shifted to neutral and the drive engine running, the friction clutch is first completely disengaged; one then waits until the speed of the transmission input shaft has fallen far enough compared with the speed of the drive engine and, finally, the friction clutch is slowly engaged until an increase in the speed of the input shaft is sensed.
The pair of values M_K1, x_K1 determined at this point in time with the known braking torque M_B of the input shaft (M_K1=M_B) and the set position x_K(M_B) is then stored as the value pair of the fixed point M_K1(x_K1) and used for the adaptation of an existing characteristic M_K′=f(x_K).
In this case, the braking torque M_B and, hence too the torque value M_K1 concerned, is relatively small and corresponds to the drag torque of the input shaft, which is caused by bearing wear, rolling losses of meshing gear wheels and oil splash losses of the input shaft and of components connected to it. In addition, the drag torque is strongly temperature-dependent and increases as the operating temperature falls, in particular the oil temperature of the transmission. The disadvantage of this known method for determining a torque characteristic are the large number of process steps needed, the relatively long duration of the process sequence, which can lead to delays during normal driving operation of the motor vehicle, and the relatively poor precision of the fixed point determinations.
The purpose of the present invention is to indicate a method for determining a torque characteristic of an automated friction clutch of the type mentioned to begin with, by way of which at least one of the marker points M_K1(x_K1), M_K2(x_K2) of the torque characteristic M_K=f(x_K) can be determined more simply, more rapidly and with greater precision.